The invention relates to an automatic transmission.
Multi-gear automatic transmissions have been known for a long time. They usually consists of planetary trains which are shifted by means of clutches and brakes and are connected via a hydrodynamic torque converter with the power train of the vehicle, especially of an internal combustion engine.
Earlier multi-gear automatic transmissions were designed as three-gear transmissions having one double planetary train and four or five clutches. There have also been produced more complex four-gear transmissions which have been constructed on the basis of the classical three-gear transmissions adding to them one simple planetary train with two reduction ratios and two clutches and brakes so that the systems have altogether one double planetary train, one simple planetary train, six or seven clutches and brakes, the same as the torque converter and a lock-up clutch.
In the meantime, five-gear and six-gear transmissions have also been developed which, in addition, have one planetary train with two reduction ratios and two clutches and brakes. This leads to multi-gear automatic transmissions with altogether one double planetary train, one simple planetary train, seven or eight clutches and brakes in addition. To the torque converter with the lock-up clutch which also require considerable space.
In the Applicant""s DE 44 15 664, a multi-gear automatic transmission with wet-running clutches and brakes in disk design has been described. Each clutch or brake consists of a first cylinder with outer disks, a second cylinder with inner disks, a piston and a recoil element, both cylinders being provided in the area of the disks with axially extending grooves so that the disks can be axially moved by the piston. The cylinder/disk torque transmission is form locking with the clutch closing while the piston presses the disks against an end stop. The force for adjusting the piston is applied by the oil contained in the transmission. The clutch opens while the action of force of the oil is broken down. By the recoil element the piston is pushed back to its original position whereby the frictional engagement of the inner and outer disks is released.
The problem on which this invention is based is to provide an automatic transmission having at least six gear steps and large spreading effect, which has a compact construction and is easily situated in vehicles having front wheel drive and transversely lying engine.
It is proposed, therefore, according to the invention, that by adequate selection and coupling of the gears of the wheel train one of the clutches of the main transmission is both the starting clutch for the motor vehicle and the gear clutch for the first gear and for the reverse gear.
The use of one clutch of the main transmission as a wet starting clutch makes the elimination of the formerly customary separate starting element possible, such as a hydrodynamic torque converter. The starting clutch assumes several functions in the transmission, in addition to its primary function as gear clutch in the change of gear, it assumes the function of starting.
The adequate selection of the wheel train so that only one and the same starting clutch can be produced, the positive engagement both for the first gear and for the reverse gear offers the advantage that only this starting clutch has to be thermally and automatically controlled in such a manner that the starting is made possible both in forward direction and in reverse direction.
The thermal and automatic control design of the inventive starting clutch is to be selected so that it bears the thermal load when starting without problem and also makes a sensitive starting possible. This ensures a great driving comfort when starting at less expense than in the conventional multi-gear automatic transmission where a hydrodynamic torque converter, for example, takes over the starting function. Rolling out shifts to the first gear require a sensitive application of the clutch torque.
As a specially advantageous inventive design of the starting clutch, it is proposed to design the clutch with two pistons so that during pressurization the second piston presses only one part of the friction disk of the starting clutch thus increasing the pressure to torque ratio. To this end, both pistons can be pressurized simultaneously or independently of each other.
An advantageous design and arrangement of the starting clutch as brake, such as described in DE-A-196 25 355, makes an easy supply of the cooling oil possible, since the oil supply does not proceed through rotating parts with the attendant expensive sealing due to rectangular rings, for example.
Since a large amount of cooling oil is needed only during the starting operation, the cooling oil supplied is advantageously controlled specifically by control of the oil amount equal in time to the real onset of heat. The latter is determined by electronic evaluation of torque and differential rotational speed of the starting clutch so that the oil supply can be designed according to requirement which results in minimizing the transmission losses. In particular, the cooling of the latter outside the slip phase of the starting clutch can be minimal, even zero. The spared cooling oil can be used for the remaining clutches and for cooling the wheel trains.